qemu/docs/qcow2-cache.txt

6 This work is licensed under the terms of the GNU GPL, version 2 or
12 performance significantly. However, setting the right cache sizes is
24 A qcow2 file is organized in units of constant size called clusters.
26 The cluster size is configurable, but it must be a power of two and
42 There is one single L1 table per disk image. The table is small and is
46 allocated in the image. Each table is one cluster in size. In order to
48 corresponding L2 table to find out where that data is located. Since
60 snapshots. The data is stored in a two-level structure similar to the
64 cluster in size and the number is also variable and dependent on the
68 is a power of two and must not be higher than 64. It defaults to 16
83 caches (in bytes) is:
100 For example, 1MB of L2 cache is needed to cover every 8 GB of the virtual
101 image size (given that the default cluster size is used):
105 The refcount cache is 4 times the cluster size by default. With the default
106 cluster size of 64 KB, it is 256 KB (262144 bytes). This is sufficient for
125  - Both caches must have a size that is a multiple of the cluster size
128  - The maximum L2 cache size is 32 MB by default on Linux platforms (enough
133    On non-Linux platforms the maximal value is smaller by default (8 MB) and
136    The minimal L2 cache size is 2 clusters (or 2 cache entries, see below).
138  - The default (and minimum) refcount cache size is 4 clusters.
140  - If only "cache-size" is specified then QEMU will assign as much
156 refcount cache is as small as possible unless overridden by the user.
162 needs an entry from an L2 table then the whole table is read from disk
163 and is kept in the cache. If the cache is full then a complete table
184    disk I/O faster. This is particularly true with solid state drives
186    rotating hard drives the situation is a bit more complicated so you
190    in your case. The block size of the host filesystem is generally a
197  - If the L2 cache is big enough to hold all of the image's L2 tables
200    this is necessary and you can omit the "l2-cache-entry-size"
207 It is possible to clean unused cache entries in order to reduce the
219 If unset, the default value for this parameter is 600 on platforms which
220 support this functionality, and is 0 (disabled) on other platforms.
223 madvise() to actually free the memory. This is a Linux-specific feature,
224 so cache-clean-interval is not supported on other systems.